1. Field of the Invention
The present invention relates to a measuring apparatus utilizing attenuated total reflection (hereinafter referred to as ATR), such as a surface plasmon resonance measuring apparatus for quantitatively analyzing a substance in a sample by utilizing excitation of surface plasmon, and more particularly to a measuring apparatus, utilizing ATR, of a type that detects a dark line occurring in a measuring light beam by ATR with the use of photodetection means consisting of light-receiving elements juxtaposed in a predetermined direction.
2. Description of the Related Art
In metals, if free electrons are caused to vibrate in a group, a compression wave called a plasma wave will be generated. The compression wave, generated in the metal surface and quantized, is called surface plasmon.
There have hitherto been proposed various kinds of surface plasmon resonance measuring apparatuses for quantitatively analyzing a substance in a sample by taking advantage of a phenomenon that surface plasmon is excited by a light wave. Among such apparatuses, one employing a system called xe2x80x9cKretschmann configurationxe2x80x9d is particularly well known (e.g., see Japanese Unexamined Patent Publication No. 6(1994)-167443).
The surface plasmon resonance measuring apparatus employing the xe2x80x9cKretschmann configurationxe2x80x9d is equipped with a dielectric block formed, for example, into the shape of a prism; a metal film, formed on one surface of the dielectric block, for placing a sample thereon; and a light source for emitting a light beam. The measuring apparatus is further equipped with an optical system for making the light beam enter the dielectric block so that a condition for total internal reflection (TIR) is satisfied at the interface between the dielectric block and the metal film and that ATR due to surface plasmon resonance can occur; and photodetection means for measuring the intensity of the light beam totally reflected at the interface, and thereby detecting surface plasmon resonance.
To obtain various angles of incidence in the aforementioned manner, a relatively thin light beam can be deflected so that it strikes the above-mentioned interface at different angles of incidence, or a relatively thick beam can be emitted convergently or divergently so that the components thereof strike the interface at various angles of incidence. In the former, the light beam whose reflection angle varies with the deflection thereof can be detected by a small photodetector that is moved in synchronization with the light beam deflection, or by an area sensor extending along a direction where the reflection angle varies. In the latter, on the other hand, the light beams reflected at various angles can be detected by an area measuring apparatus extending in a direction where all the reflected light beams are received.
In the surface plasmon resonance measuring apparatus mentioned above, an evanescent wave having electric field distribution is generated in a sample in contact with the metal film, if a light beam strikes the metal film at a specific incidence angle xcex8sp greater than a critical incidence angle at which total internal reflection (TIR) takes place. The generated evanescent wave excites surface plasmon at the interface between the metal film and the sample. When the wave vector of the evanescent wave is equal to the wave number of the surface plasmon and therefore the wave numbers between the two are matched, the evanescent wave resonates with the surface plasmon and the light energy is transferred to the surface plasmon, whereby the intensity of the light satisfying TIR at the interface between the dielectric block and the metal film drops sharply. This sharp intensity drop is generally detected as a dark line by the above-mentioned photodetection means.
Note that the above-mentioned resonance occurs only when an incident light beam is a p-polarized light beam. Therefore, in order to make the resonance occur, it is necessary that a light beam be p-polarized before it strikes the interface.
If the wave number of the surface plasmon is found from the specific incidence angle xcex8sp at which ATR takes place, the dielectric constant of a sample to be analyzed can be calculated by the following Equation:
xe2x80x83Ksp(xcfx89)=(xcfx89/c){∈m(xcfx89) ∈s}1/2/{∈m (xcfx89)+∈s}1/2
where Ksp represents the wave number of the surface plasmon, xcfx89 represents the angular frequency of the surface plasmon, c represents the speed of light in vacuum, and ∈m and ∈s represent the dielectric constants of the metal and the sample, respectively.
If the dielectric constant ∈s of a sample is found, the density of a specific substance in the sample is found based on a predetermined calibration curve, etc. As a result, the dielectric constant of the sample, i.e., the properties of the sample related to the refractive index thereof can be quantitatively analyzed by finding the specific incidence angle xcex8sp at which the intensity of the reflected light at the interface drops sharply.
In this kind of surface plasmon resonance sensor, photodetection means in the form of an array can be employed with the object of measuring the aforementioned incidence angle xcex8sp with a high degree of accuracy and in a large dynamic range, as disclosed in Japanese Unexamined Patent Publication No. 11(1999)-326194. The photodetection means is constructed of a plurality of light-receiving elements juxtaposed in a predetermined direction. The light-receiving elements are disposed to respectively receive the components of a light beam reflected at the aforementioned interface at various angles of reflection.
In that case, differentiation means is provided to differentiate the photodetection signals output by the light-receiving elements of the aforementioned photodetection means, in the direction where the light-receiving elements are juxtaposed. The properties of the sample related to the refractive index thereof are often analyzed based on differentiated values output by the differentiation means, particularly the differentiated value corresponding to a dark line that occurs in a reflected light beam. The differentiation means can employ, for example, means for detecting the difference between the outputs of two adjacent light-receiving elements.
In the case of detecting the properties of a sample corresponding to the aforementioned incidence angle xcex8sp by obtaining the difference between the outputs of two adjacent light-receiving elements, it is possible to employ a two-piece photodiode instead of the array-shaped photodetection means, if a large dynamic range is not required. In that case, the difference between the outputs of two photodiodes corresponds to the position of the aforementioned dark line in the direction where the photodiodes are arranged. Based on the difference, the properties of a sample corresponding to the aforementioned incidence angle xcex8sp can be detected.
On the other hand, as a similar resonance measuring apparatus making use of ATR, there is known a leaky mode sensor (e.g., see xe2x80x9cSpectral Researches,xe2x80x9d Vol. 47, No.1 (1998), pp. 21 to 23 and pp. 26 to 27). This leaky mode measuring apparatus is equipped with a dielectric block formed, for example, into the shape of a prism; a cladding layer formed on one surface of the dielectric block; and an optical waveguide layer, formed on the cladding layer, for placing a sample thereon. The leaky mode sensor is further equipped with a light source for emitting a light beam; an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection (TIR) is satisfied at the interface between the dielectric block and the cladding layer and so that ATR occurs by a waveguide mode excited in the optical waveguide layer; and photodetection means for measuring the intensity of the light beam totally reflected at the interface between the dielectric block and the cladding layer, and thereby detecting the excited state of the waveguide mode, that is, ATR.
In the leaky mode measuring apparatus mentioned above, if a light beam strikes the cladding layer through the dielectric block at incidence angles greater than a critical incidence angle at which TIR takes place, the light beam is transmitted through the cladding layer and then only light with a specific wave number, incident at a specific incidence angle, propagates through the optical waveguide layer in a waveguide mode. If the waveguide mode is excited in this manner, the greater part of the incident light is confined within the optical waveguide layer, and consequently, ATR occurs in which the intensity of light totally reflected at the above-mentioned interface drops sharply. Since the wave number of the light propagating through the optical waveguide layer depends on the refractive index of a sample on the optical waveguide layer, both the refractive index of the sample and the properties of the sample related to the refractive index thereof can be analyzed by finding the above-mentioned specific incidence angle xcex8sp at which ATR takes place.
The leaky mode measuring apparatus can also employ the aforementioned array-shaped photodetection means or two-piece photodiode, because the apparatus detects the position of a dark line which occurs in a reflected light beam by ATR.
However, in the case where the aforementioned array-shaped photodetection means is employed in a measuring apparatus utilizing ATR, such as the aforementioned surface plasmon resonance measuring apparatus and leaky mode measuring apparatus, there is a problem that accuracy of measurement will be reduced.
The present invention has been made in view of the circumstances mentioned above. Accordingly, it is the primary object of the present invention to provide a measuring apparatus, utilizing ATR, which is capable of assuring high accuracy of measurement.
To achieve this end and in accordance with the present invention, there is provided a first measuring apparatus utilizing attenuated total reflection, comprising:
a dielectric block;
a thin film layer, formed on one surface of the dielectric block, for placing a sample thereon;
a light source for emitting a light beam;
an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection is satisfied at an interface between the dielectric block and the thin film layer;
photodetection means, comprising a plurality of light-receiving elements juxtaposed in a predetermined direction and disposed to respectively receive components of the light beam totally reflected at the interface, for detecting attenuated total reflection from a dark line occurring in the totally reflected light beam;
movement means for moving the photodetection means in the direction where the plurality of light-receiving elements are juxtaposed;
position detection means for detecting position of the photodetection means moved by the movement means;
storage means for storing the position of the photodetection means detected by the position detection means; and
control means for controlling the movement means when the attenuated total reflection is detected a plurality of times for a single sample, and then disposing the photodetection means at a predetermined position relative to the dark line when a first measurement is made and also disposing the photodetection means at the same position as the predetermined position stored in the storage means when a second measurement and measurements thereafter are made.
In accordance with the present invention, there is provided a second measuring apparatus utilizing attenuated total reflection, comprising:
a dielectric block;
a metal film, formed on one surface of the dielectric block, for placing a sample thereon;
a light source for emitting a light beam;
an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection is satisfied at an interface between the dielectric block and the metal film;
photodetection means, comprising a plurality of light-receiving elements juxtaposed in a predetermined direction and disposed to respectively receive components of the light beam totally reflected at the interface, for detecting attenuated total reflection due to surface plasmon resonance from a dark line occurring in the totally reflected light beam;
movement means for moving the photodetection means in the direction where the plurality of light-receiving elements are juxtaposed;
position detection means for detecting position of the photodetection means moved by the movement means;
storage means for storing the position of the photodetection means detected by the position detection means; and
control means for controlling the movement means when the attenuated total reflection is detected a plurality of times for a single sample, and then disposing the photodetection means at a predetermined position relative to the dark line when a first measurement is made and also disposing the photodetection means at the same position as the predetermined position stored in the storage means when a second measurement and measurements thereafter are made.
In accordance with the present invention, there is provided a third measuring apparatus utilizing attenuated total reflection, comprising:
a dielectric block;
a cladding layer formed on one surface of the dielectric block;
an optical waveguide layer, formed on the cladding layer, for placing a sample thereon;
a light source for emitting a light beam;
an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection is satisfied at an interface between the dielectric block and the cladding layer;
photodetection means, comprising a plurality of light-receiving elements juxtaposed in a predetermined direction and disposed to respectively receive components of the light beam totally reflected at the interface, for detecting attenuated total reflection due to a waveguide mode excited in the optical waveguide layer, from a dark line occurring in the totally reflected light beam;
movement means for moving the photodetection means in the direction where the plurality of light-receiving elements are juxtaposed;
position detection means for detecting position of the photodetection means moved by the movement means;
storage means for storing the position of the photodetection means detected by the position detection means; and
control means for controlling the movement means when the attenuated total reflection is detected a plurality of times for a single sample, and then disposing the photodetection means at a predetermined position relative to the dark line when a first measurement is made and also disposing the photodetection means at the same position as the predetermined position stored in the storage means when a second measurement and measurements thereafter are made.
In the three measuring apparatuses, the photodetection means may be constructed of a two-piece photodiode or photodiode array.
The inventors have made various investigations and experiments with respect to a mechanism causing a reduction in accuracy of measurement when employing array-shaped photodetection means in the conventional measuring apparatus utilizing ATR, and found that the reduction in accuracy of measurement results from a difference in physical characteristics among a plurality of light-receiving elements constituting the photodetection means.
That is, since the aforementioned specific incidence angle (at which ATR occurs) varies from sample to sample, a light-receiving element of a plurality of light-receiving elements which detects a dark line also varies with a sample. In surface plasmon resonance measuring apparatuses and leaky mode measuring apparatuses, the properties of a sample are analyzed by detecting the position of the light-receiving element detecting the dark line. Therefore, if there is a difference in physical characteristics among a plurality of light-receiving elements, the difference will have influence on the detection of the dark line and reduce accuracy of measurement.
In view of the above-mentioned facts, the measuring apparatus of the present invention is provided with movement means for moving photodetection means in the direction where a plurality of light-receiving elements are juxtaposed, position detection means for detecting position of the photodetection means moved by the movement means, and storage means for storing the position of the photodetection means detected by the position detection means. The measuring apparatus is further provided with control means for controlling the movement means when the attenuated total reflection is to be detected a plurality of times for a single sample. The photodetection means is disposed at a predetermined position relative to the dark line when a first measurement is made, and also the photodetection means is disposed at the same position as the predetermined position stored in the storage means when a second measurement and measurements thereafter are made.
With this arrangement, the relative position of the photodetection means with respect to the dark line position can be maintained constant without depending on the aforementioned specific incidence angle xcex8sp, i.e., a sample. In this state, a plurality of measurements can be started. Therefore, even when there is a difference in physical characteristics among a plurality of light-receiving elements constituting array-shaped photodetection means, the influence of the difference in physical characteristics on the detection of the dark line position becomes constant for each measurement and therefore there is no possibility that accuracy of measurement will be reduced due to the difference in physical characteristics.